1. Field of the Invention
The present invention relates to a light guide plate, a surface light source and a display using the light guide plate, and more particularly, relates to a light guide plate which collects light from an internal light source and external light from the sun or a fluorescent lamp and emits a simple light or a synthesized light from a light emissive surface thereof, a surface light source serving as a back light for a non-self light emission type display, which does not emit light by itself, such as a liquid crystal display, and a back light type display such as a liquid crystal display for a digital still camera or a video camera which displays an object.
2. Description of the Prior Art
Examples of a surface light source for a back light of a liquid crystal display (LCD) include one of a side light type having a transparent flat plate as a light guide plate. In such a surface light source, light from such light sources as a fluorescent lamp is incident from one of side end surfaces of a light guide plate comprising a transparent parallel flat plate or a flat plate having a wedge shape in cross section, the light is propagated throughout the whole area of the light guide plate by utilizing total reflection inside the transparent flat plate, a part of the propagated light is changed into diffused reflected light at less than a critical angle by a light scattering reflective plate on the reverse surface of the light guide plate, and the diffused light is emitted from the surface of the light guide plate (see Japanese Utility Model Laid-Open No. 16220 1/1980).
Furthermore, examples of a surface light source for a back light include one that has an external light collecting portion at side end surfaces of the light guide plate in order to use external light such as the sunlight and light from a lighting apparatus as a secondary light source. The external light collecting portion, having a semicircle shape in cross section, forms a cylindrical lens (see Japanese Patent Laid-Open No. 11249/1993).
FIG. 6 is a schematic view illustrating a conventional liquid crystal display.
The conventional liquid crystal display comprises a liquid crystal panel 111, an internal light source 110 such as a fluorescent tube, and a light guide plate 100. When using this liquid crystal display as a liquid crystal monitor for an electric still camera, for example, a body of a camera is positioned on the left hand of the light guide plate 100 in the figure, and a user watches images from the right hand of the liquid crystal panel 111 in the figure through the light emitted from the light emissive surface 103.
An external light collecting portion 101 comprising a cylindrical lens having a semicircle shape in cross-section is provided at an upper end of the light guide plate 100 included in the conventional liquid crystal display in order to collect external light as main light. Therefore, external light from the upper and side direction can be collected effectively.
An internal light source 110 is provided in the lower proximity to an internal light collecting surface 102 which is opposite to the external light collecting portion 101. The internal light source 110 emits secondary light to the internal light collecting surface 102 when external light is not sufficient due to cloudy or rainy weather or indoor use. The light incident from the internal light collecting surface 102 reflects several times in the inside of the light guide plate 100 and most of the incident light is emitted from the light emissive surface 103 in the end.
The conventional liquid crystal display, however, has a construction in which the external light collecting portion 101 and the internal light source 110 face each other. In conjunction with this, light reflection process can not be applied to the external light collecting portion 101 and the internal light collecting surface 102 since light comes in from the external and internal light source. As a result, a part of the external light collected from the external light collecting portion 101 leaks from the internal light collecting surface 102 and a part of the light incident from the internal light source through the internal light collecting surface 102 e dispersed from the external light collecting portion 101, causing extreme decrease the efficiency of utilizing the external light and the light from the internal light source.
The thickness of the light guide plate comprising the light guide plate 100 and the internal light source 110 of the conventional liquid crystal display in FIG. 6 is considerably restricted to the thickness of the internal light source 110 as taking the collecting efficiency of internal light and limitation in size of the internal light source 110 in mechanical view point into consideration. It causes a difficulty in making a light guide plate slimmer.